For a transmission line, there generally exist two types of faults, i.e. a permanent fault and a temporary fault, in which about 90% of the faults are single-phase faults, and over 80% of the single-phase faults are temporary faults. Auto-reclosing the circuit breaker (CB) is a generally effective way to improve the stability and continuity of a power system. However, if a transmission line with a permanent fault is auto-reclosed, some risks may be arisen, for example the primary devices may be damaged, device insulation can be destroyed, the system stability may be threatened, and no continuous current can be supplied stably. So it is very important and desirable for customers to distinguish temporary faults from permanent faults in a transmission line.
At present, some methods have been proposed to distinguish these two kinds of faults to avoid auto-reclosing the CB in the case of permanent fault occurrence. Among them, the method based on the mutual voltage (i.e. capacitive coupling voltage) is extensively adopted, whose basic principle can be described as following.
FIG. 1 illustrates a single-phase fault (opened phase A) occurs in a transmission. In FIG. 1, for example, phase A is opened at both ends of the conductor. When the fault phase of the transmission line is isolated, the isolated phase conductor still possesses a voltage to the ground, i.e. a capacitive coupling voltage, and an inductive voltage from the other two unbroken healthy phase conductors (for example phase B and phase C).
The steady state voltage Uy caused by capacitive coupling can be calculated according to the following equation (1):
                              U          ⁢                                          ⁢          y                =                                                            b                0                            -                              b                1                                                                    2                ⁢                                  b                  1                                            +                              b                0                                              ⁢                                                    U                *                            A                        .                                              (        1        )            
In which b0 and b1 are the zero sequence and positive sequence capacitive susceptance per unit line length of the transmission line respectively; {dot over (U)}A represents phase A voltage vector.
The inductive voltage UXL on the opened phase A conductor can be calculated according to the following equation (2):{dot over (U)}XL=(İB+İC)ZmL=(İB+İC)(Z0−Z1)L/3={dot over (U)}XL  (2)
in which IB and IC are the currents of the unbroken healthy phase conductors phase B and C respectively; Z0 and Z1 are the zero sequence and positive sequence impedance of the protected transmission line respectively; Zm is the mutual inductance impedance per unit line length; L is the length of the transmission line; UX represents inductive voltage per unit line length.
It's obvious to the skilled person that the capacitive coupling voltage Uy is the voltage of phase A to ground, and independent of the line length and the load. While the inductive voltage UXL is longitudinal along the conductor and proportional to the load current and line length. Then the terminal voltage UAMT and UANT magnitudes on the opened phase conductor can be calculated according to the following equation (3) and (4) respectively:UAMT=√{square root over (Uy2+(UXL/2)2−UyUXL cos(θ+π/2))}  (3)UANT=√{square root over (Uy2+(UXL/2)2−UyUXL cos(−θ+π/2))}  (4).
In which θ is power factor and subscript ‘T’ means temporary fault.
FIG. 2a illustrates an equivalent circuit diagram about the temporary fault voltage on the opened phase conductor terminals, and FIG. 2b illustrates the voltage vector diagram about the temporary fault voltage on the opened phase conductor terminals.
According to above principle, the distinguishing method based on the mutual voltage (i.e. capacitive coupling voltage) offers three criterions as below:
1) Voltage criterion (if Uy≥k1*UMXL):U≥K1UMXL  (5).
In which U is the voltage value measured at the end of opened phase conductor; K1 is a reliability coefficient; and UMXL, is the inductive voltage UMXL at the maximum load condition.
This inequality (5) means that if the measured voltage U is above or equivalent to the predefined threshold, a transient fault can be determined.
2) Compensation voltage criterion (if k1*UMXL>Uy>=k1*UMXL/2).
It will be obvious to the skilled person that the inductive voltage is dependent on the load current and length of the line from the equation (2). For a long transmission line with heavy load, previous voltage criterion 1 will represent an unsatisfied performance, that's to say, the criterion 1 cannot distinguish the fault; consequently a revised criterion 2-compensation criterion is described as follow:
                    |                  U          -                                    U              MXL                        2                          |                  ≥                      |                                                            K                  2                                ⁢                                  U                  MXL                                            2                        |            .                                              (        6        )            
In which K2 is a reliability coefficient; and when this inequality (6) comes into existence in the case of a long transmission line with heavy load, the fault is determined as a transient fault.
3) Composed voltage criterion (if k1*UMXL/2>Uy):
The composed voltage criterion 3 can be described as follow:
                                                                        |                                  U                  -                                                            U                      MXL                                        4                                                  |                                  >=                                      |                                                                                            K                          3                                                ⁢                                                  U                          MXL                                                                    4                                        |                                                                                                                          |                                  U                  -                                                            3                      *                                              U                        MXL                                                              4                                                  |                                  >=                                      |                                                                                            K                          3                                                ⁢                                                  U                          MXL                                                                    4                                        |                                                                                      .                            (        7        )            
In which K3 is a reliability coefficient; and when these two inequalities are met simultaneously, the transient fault is identified.
According to the prior arts about the reclosing technique, existed solutions including above mentioned method generally represents good performance in distinguishing a permanent fault from a transient fault in the majority of operational conditions, but for some special conditions such as: the fault in a short transmission line, the fault with high fault resistance or the fault with heavy load, its performance will be degraded and the fault identification will be inaccurate even wrong. That means such drawback decreases the reliability of adaptive re-closure technique, and customers cannot be thoroughly convinced of utilizing this adaptive re-closure strategy to reclosing the CBs safely.